1. Field of the Invention
The present invention generally relates to apparatus and methods for measuring pollutants in an exhaust and more particularly to an apparatus and method to determine the concentration of particles in a combustion exhaust stream.
2. Discussion of the Background
Combustion processes generally produce gaseous and particulate species as by-products. In particular, the combustion of carbon-containing fuels such as petroleum products; bio-derived liquid fuels; coal; and biomass such as wood, may create carbonaceous particles. These particles may include “black carbon” (or BC), which is implicated in local, regional and global climate change, due to their ability to absorb sunlight and change the properties of clouds; and which are also well-known tracers for adverse human health impacts arising from their inhalation and deposition in the lungs and body tissues.
The emission of black carbon particles is highly variable from one source to another, and can not be predicted: the emissions must be measured. Black carbon particulate emission is found to depend strongly on the type of fuel; the quality of combustion; and, for internal combustion engine sources, the degree of maintenance of the engine. The ability to make a rapid and accurate measurement of the concentration of black carbon in an exhaust stream is therefore highly advantageous for determining a source's emissions strength if required to assess potential health risk; and for examining the emissions from a suite or set of sources such as a fleet of vehicles to permit determination of compliance with an emissions standard, or as a means of determining which engines require remedial attention.
The measurement of emissions in the exhaust of a combustion source such as a diesel engine, a chimney, or a wood- or coal-burning stove generally requires conditioning of the sample before being amenable to real-time analysis for particulate content by existing analytic methods. The particles in the exhaust stream are typically very small in size (often referred to as “Ultrafine Particulate Matter”) and have a high affinity for coagulating by accumulation into a smaller number of slightly larger particles. This tends to confound the analysis by optical single particle counters, or devices based upon measuring the reflection or scattering of light from the particles, since the reflection and scattering of light is a very strong function of the particle size. Coagulation of ultrafine particles into small particles confounds measurement methods based upon optical scattering or size determination. The exhaust stream also typically contains a high content of water vapor, derived from the combustion of the hydrogen in the hydrocarbon fuel to form water. This humidity will moisten any collection substrate and usually thereby contribute a substantial error to subsequent analysis. Finally, the exhaust stream is usually hot, potentially causing problems for an analytical instrument if the stream is passing through the instrument continuously.
For these reasons, conventional real-time analytical techniques for the study of the particulate content of direct combustion exhaust streams almost always dilute the exhaust sample continuously with a large excess of filtered, particle-free, dry injection air at room temperature. Dilution air is usually added in a ratio from 10:1 to 100:1 relative to the exhaust sample. Dilution separates the ultrafine particles, reducing coagulation of the particles. Dilution also reduces the net relative humidity, reducing errors when analyzed by collection-based techniques. Dilution further reduces the temperature from hot exhaust to close to room temperature, making it more suited to analysis by optical or electronic components.
The use of dilution also represents a substantial need for additional equipment such as pumps, flow controllers, valves and a mixing chamber; supported, in turn, by supplies of dry compressed air and other incidentals. Thus dilution-based real-time analysis of exhaust particulates is neither portable nor convenient to use for measuring emissions at their source.
There is a need for apparatus and methods that allow for obtaining a rapid sample of the direct, undiluted exhaust emissions stream and performing an instantaneous analysis for black carbon content using analytical techniques. Such an apparatus and method should be easy to use and inexpensive, and not require the use of specialty diluent gas or excessive pumps, valves, and components for diluting a sample. The apparatus should be portable, should not require supporting infrastructure such as the provision of dry compressed air; and should be able to collect and analyze a sample in a short time, such as less than 30 seconds.